This invention relates to means for producing gas throughout a mass of synthetic resin, and to resultant synthetic resin products.
The production of gas in resin has a number of uses. By way of example, gas can be generated to produce a foamed cellular product (by action of a blowing agent added to the resin), or to ensure the filling of a mold cavity or to prevent shrinking during curing of such a product (use of an agent for anti-sink or anti-shrink purposes). There has long been active interest in finding improved gas generation methods, particularly for polyester resins.
Specifically, in systems using chemical means to generate gas within the resin, it is desirable to have a simple, reproducible means of gas generation which does not require external addition of heat and which does not interfere with the product. For example, systems requiring the addition of water or acid may be deleterious to the curing and strength of the product in free-radical polymerizations. Ideally, a chemical gas generating system would be inexpensive and require inexpensive equipment, would utilize components that are nontoxic and have a long shelf life (both individually and when mixed with other resin system components), would be capable of achieving a full range of foam densities, and would permit a controlled induction period after mixing so that the reaction system can be poured into a mold or otherwise formed before gas generation takes place. Capability for loading foams with fillers and reinforcements is also desirable. Finally, it is desirable that such gas-generating components be capable of functioning over a non-critical range of proportions without harming or altering the composition of the product. Unfortunately, to date, for many resin systems, the previously known blowing agents are inadequate in a number of these respects.
With regard in particular to polyester resins, there has been no wholly satisfactory foaming method. For this reason the extremely good properities of polyesters--e.g., high strength, resistance to heat distortion and, with fire retardants, relatively low flammability--have been unavailable for many applications. Azo foaming systems are expensive, explosive, toxic and require refrigeration and other special handling. Other known systems are not capable of achieving low density and, therefore, result in expensive products, and produce gases which, for temperature insulation purposes, have poor thermal qualities. Still other proposed foaming systems for polyester, e.g., those employing a urethane reaction, have produced amide, urea or urethane linkages which are undesirable in many polyester applications. Such systems have also preferably required high hydroxyl-content polyester resins, which result in shortened pot life.
In addition to the need for improved gas-generating means or blowing agents in general, and for better agents for use with polyester resins in particular, there is also a need for new resin systems that have improved physical properties from the point of view of cost-benefit, such as better combinations of strength, lack of friability, better heat resistance and better sound and thermal insulating properties.